Light pickup

ABSTRACT

An optical pickup capable of recording or reproducing information from optical disks having different recording densities includes a first optical unit having a light source which emits rays having a first wavelength for DVDs and a first light sensor which detects reflected rays, a second optical unit having a light source which emits rays having a second wavelength for CDs and a second light sensor which detects reflected rays, a beam splitter which directs the rays having the first wavelength and the rays having the second wavelength along substantially the same optical axis, and directs reflected rays from an optical disk to the first light sensor and the second light sensor, and an objective lens which has a numerical aperture (NA) of 0.6 or more, a focal length of 2.5 mm or less, an effective diameter of incidence of 3.0 mm or less and a working distance of 1.2 mm or less. The light pickup as a whole has a thickness of 7.5 mm or less as measured from a bottom surface of the optical disk. The light pickup is capable of reproducing information from optical disks having different recording densities, is composed of a small number of parts, and is compact and thin.

This is a continuation of application Ser. No. 08/989,897 filed Dec. 12,1997, now U.S. Pat. No. 5,986,994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light pickup which is used forrecording or reproducing information from recording media havingdifferent recording densities such as high density optical disks,compact disks and so on.

2. Description of the Related Art

A conventional light pickup for recording and reproducing informationfrom high density optical disks and low density optical disks will bedescribed below. For descriptive convenience, a DVD (digital video disk)and a CD (compact disk) are taken as examples of the high densityoptical disks and the low density optical disks.

FIGS. 11A and 11B, respectively, are a plan view of the conventionallight pickup and a sectional view illustrating main components thereof.In FIGS. 11A and 11B, the reference numeral 50 represents a light pickupfor high density optical disks wherein an objective lens 54 for a highdensity optical disk which condenses a laser beam 53 onto a high densityoptical disk 52 is cemented and fixed to an objective lens holdingcylinder 51. Further, a coil unit 55 which consists of a focus coil anda tracking coil for moving the objective lens holding cylinder 51 in afocusing direction and a tracking direction is cemented and fixed to theobjective lens holding cylinder 51. On the other hand, the coil unit 55is fitted in a permanent magnet and composes a magnetic circuit fordriving the objective lens holding cylinder 51 in the focusing directionand the tracking direction. The objective lens holding cylinder 51 isheld in a neutral position with an electrically conductive nonmagneticlinear elastic member 57 for supplying electric power to the coil unit55. The objective lens for high density optical disk has a numericalaperture on the order of 0.6 and a focal length on the order of 3.3 mm.

Explanation will be made of an optical system of the light pickup 50 forhigh density optical disks which has the configuration described above.The reference numeral 61 designates an optical unit for high densityoptical disks which comprises a light emitting element and a lightreceiving element for the laser beam 53 having wavelengths of 635 to 650nm. The laser beam 53 passes through a collimator lens 62, is madeparallel, completely reflected by a surface of a raising mirror 63coated with multiple layers and condensed by the objective lens 54 forhigh density optical disks, and forms an optical spot on the highdensity optical disk 52.

Then, the laser beam 53 which is reflected by the high density opticaldisk 52 is incident again onto the optical unit 61 for high densityoptical disks via the path in a direction reverse to that describedabove, allowed to pass through a diffraction grating (not shown) andreceived by a light receiving element (not shown). On the basis ofoptical information which is subjected to photoelectric conversion bythe light receiving element, a focus is detected by the known hologramFoutcault and a track is detected by the phase difference method. Theobjective lens 54 for high density optical disks is always focused onthe high density optical disk 52 and controlled so as to follow aninformation track as described above.

The high density optical disk (DVD) 52 is rotatingly driven by a spindlemotor 71.

Then, description will be made of a light pickup for low density opticaldisks (CD). The reference numeral 70 represents a light pickup for thelow density optical disks, which will not be described in particularsince it is has a configuration and function which are similar to thoseof the light pickup 50 for high density optical disks. In an opticalsystem of the light pickup for low density optical disks, the referencenumeral 64 designates an optical unit for low density optical diskswhich comprises a light emitting element and a light receiving elementfor a laser beam 65 which has a wavelength of 780 nm. The laser beam 65is completely reflected by a surface of a raising mirror 66, condensedby an objective lens 67 for low density optical disks and forms anoptical spot on a low density optical disk 68.

Then, the laser beam 65 which is reflected from the low density opticaldisk 68 is incident again on the optical unit 64 for low density opticaldisks via the path in a direction reverse to that described above,allowed to pass through the diffraction grating and received by thelight receiving element. On the basis of optical information which issubjected to photoelectric conversion by the light receiving element, afocus is detected by the known hologram Foucault method and a track isdetected by the three-beam method. The objective lens 67 for low densityoptical disks is always focused on the low density optical disk 68 andcontrolled so as to follow an information track.

The conventional light pickup is configured so as to be capable ofrecording or reproducing information from an optical disk with the lightpickup 50 for high density optical disks and the light pickup 70 for lowdensity optical disks which are composed independently and separately asdescribed above.

However, the conventional light pickup which has the configurationdescribed above poses a problem that it requires a large number ofparts, and makes it difficult to configure the light pickups moresmall-sized and thinner since the light pickup uses two independentoptical systems for the light pickup for high density optical disks andthe light pickup for low density optical disks. Further, workingdistances of the two objective lenses described above may be set at 1.5mm or longer dependently for the sake of optical characteristics andfocus control thereof and such long working distances constitute ahindrance to more small-sized configuration of the light pickups.

A primary object of the present invention is to provide a light pickupwhich is capable of recording or reproducing information from opticaldisks having different recording densities, and configured to be compactand thin.

SUMMARY OF THE INVENTION

The light pickup according to the present invention is characterized inthat it comprises a first light source which emits rays having a firstwavelength for DVDS, a first optical sensor which detects reflected raysfrom an optical disk, a second light source which emits rays having asecond wavelength for CDs, a second optical sensor which detectsreflected rays from an optical disk, optical path modifying means whichdirects the rays having the first wavelength and the rays having thesecond wavelength along substantially the same optical axis, and directsthe reflected rays from the optical disk to the first light sensor andthe second light sensor, and an objective lens which has a numericalaperture (NA) of 0.6 or more and a focal length of 2.5 mm or less forthe rays having the first wavelength, and focuses the rays having thefirst wavelength and the rays having the second wavelength onto theoptical disk, that the objective lens has an effective diameter ofincidence of 3.0 mm or less and a working distance of 1.2 mm or less,and that the light pickup as a whole has thickness of 7.5 mm or less asmeasured from a bottom surface of the optical disk.

Further, by setting the refractive characteristic of the objective lensand the effective diameters of incidence of the rays properly the lightpickup according to the present invention can have a working distance of1.4 mm or less for the rays having the first wavelength and a workingdistance of 1.2 mm or less for the rays having the second wavelength.

By selecting the configuration described above, it is possible toprovide a light pickup which is capable of recording or reproducinginformation from optical disks having different recording densities,composed of a small number of parts, and compact and thin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a first embodimentof the light pickup according to the present invention;

FIG. 2 is a sectional view taken along a II--II line in FIG. 1;

FIGS. 3A and 3B are diagrams illustrating a shape of a diffractiongrating used in the first embodiment of the present invention;

FIGS. 4A and 4B are diagram illustrating a configuration of a lightreceiving element used in the first embodiment of the present invention;

FIG. 5 is a diagram illustrating a configuration of a second embodimentof the light pickup according to the present invention;

FIG. 6 is a sectional view taken along a VI--VI line in FIG. 5;

FIG. 7 is a diagram illustrating a configuration of a third embodimentof the light pickup according to the present invention;

FIG. 8 is a sectional view taken along a VIII--VIII line in FIG. 7;

FIG. 9 is a diagram illustrating a configuration of a fourth embodimentof the light pickup according to the present invention;

FIG. 10 is a sectional view taken along a X--X line in FIG. 9; and

FIGS. 11A and 11B are a plan view of a conventional light pickup and asectional view showing main parts thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will be described withreference to the accompanying drawings. FIG. 1 shows a configuration ofa light pickup in the first embodiment of the present invention. In FIG.1, the reference numeral 4 represents an optical unit. The optical unit4 is composed of a light source 2 emitting rays for high density opticaldisks, a light receiving element 3 for receiving rays reflected by ahigh density optical disk, etc. which are mounted on a base plate, aside wall formed as an optical path which surrounds these members, alight outgoing section formed as an open window and so on.

It is preferable that the light source 2 has an oscillating wavelengthof 800 nm or less so that a beam spot formed by the rays which areemitted from the light source and converged onto a recording medium caneasily be adjusted to a size on the order of a pitch of a track formedon the recording medium. When the light source is to be used forrecording and reproducing information from high density optical disks inparticular, it is further preferable that the light source has anoscillating wavelength of 650 nm or less since such a light source canform a beam spot small enough to reproduce information recorded at veryhigh densities on recording media, thereby making it possible to easilyobtain storage means having large capacities. On a premise that thelight source is to be used for reproducing information from DVDs, thefirst embodiment uses a light source which has wavelengths on the orderof 600 to 680 nm, or within a range from 630 to 660 nm in particular.

An optical member 5 is cemented to a light outgoing section of theoptical unit 4. This optical member 5 has a role to direct rays emittedfrom the light source 2 and reflected by a recording medium to apredetermined location of the light receiving element 3. In thisembodiment, returning rays are directed with a diffraction grating 5aformed on the optical member 5.

The optical member 5 is formed of a transparent plate-like member, and adiffraction grating 5a (FIG. 3a) for dividing an optical path is formedon at least one of the surfaces which intersect nearly perpendicularlyan optical axis of outgoing rays. It is preferable that the opticalmember 5 as a whole is formed as a plane parallel plate so that it canprevent generation of aberrations, thereby favorably reproducing signalsor forming tracking signals. Further, it is possible to preventgeneration of astigmatism and degradation of reproduced signals due toblurring of a beam spot by disposing the optical member 5 so that itstop surface and bottom surface are accurately perpendicular to anoptical axis of transmitting rays.

The reference numeral 8 represents an optical unit. The optical unit 8comprises a light source 6 which emits rays for a low density opticaldisk and a light receiving element 7 which receives rays reflected bythe low density optical disk, etc. mounted on a base plate, a side walldisposed so as to surround these members and so on. Differences betweenthe optical unit 8 and the optical unit 4 will be described below.

It is preferable that the light source 6 has an oscillation wavelengthof 800 nm or less so that a beam spot formed with rays emitted from thelight source and converged onto a recording medium can easily beadjusted to a size on the order of a pitch of tracks formed on therecording medium. A light source having an oscillating wavelength longerthan that of the light source 2 can be used as the light source 6 andwhen information on a CD is to be reproduced, for example, a lightsource having an oscillating wavelength on the order of 780 nm can forma beam spot having a sufficient size on the low density optical disk.

An optical member 9 is constituted in a similar manner to that of theoptical member 5. However, a diffraction grating 9a (FIG. 3b) formed onthe optical member 9 may be different from the diffraction grating 5a ina certain case and will be described below. An arrangement of the lightreceiving sections on the light receiving element 7 is often differentfrom the arrangement of the light receiving sections on the lightreceiving element 3. When a focus error signal, etc. are to be generatedby the diffraction grating 9a at a stage to direct rays from an opticaldisk to the light receiving element 7, it is preferable to configure thediffraction grating 9a so that it has a shape which is different fromthat of the diffraction grating 5a and generates signals optimum forindividual optical disks, thereby making it possible to obtain a highlyreliable light pickup which can perform more accurate signal generationand operation control with less errors.

When a tracking control is to be performed in particular by thethree-beam method, a diffraction grating is disposed as a beamgenerating section 9d on the light source side of the optical member 9.The beam generating section 9d is disposed at a location where itpermits the outgoing rays to transmit therethrough, and it is clear ofan optical path for rays which are reflected by the disk, diffracted bythe diffraction grating 9a and travel toward the light receiving element7.

The light pickup according to the first embodiment using the two lightsources as described above has excellent characteristics describedbelow. That is, the light pickup is capable of forming beam spots whichare suited for different kinds of recording media as described above.Further, the light pickup is capable of setting wavelengths of the lightsources at levels matched with recording characteristics of therecording media. Furthermore, the light pickup allows setting ofeffective diameters of incidence of the objective lens and the rays sothat the rays are focused on information recording layer depths in theDVD and CD in the above example which have different depths as measuredfrom the bottom surfaces.

The reference numeral 10 represents a beam splitter which acts to directtwo laser beams from the optical unit 4 and the optical unit 8 to theoptical disk, and to direct reflected rays from the optical disk to thelight receiving elements 3 and the light receiving element 7. The beamsplitter 10 has such properties as to reflect the rays from the opticalunit 4 at a high ratio and transmit the rays from the optical unit 8 ata high ratio.

The reference numeral 16 designates an objective lens which is usablefor recording or reproducing information commonly on the high densityoptical disk and low high density optical disk. This objective lens hasa numerical aperture of 0.6 or more which is required for recording orreproducing information from the high density optical disk (i.e., forrays having the wavelength of the light source 2) and a focal length of2.5 mm or less for the high density optical disk or a working distance(a distance as measured from an outgoing side end surface of theobjective lens 16 to the bottom surface of the optical disk: thisapplies also to the following description) of 1.2 mm or less for thehigh density optical disk. A light bundle incident on the objective lens16 for recording or reproducing the information from the optical diskmust have a diameter two times or more than a product of the numericalaperture multiplied by the focal length (a distance as measured from acenter of the objective lens to a focal point thereof when parallel raysare incident onto a side surface of the objective lens and rays outgoingfrom the other side surface are focused: this applies also to thefollowing description). The diameter of the light bundle directlycontributes directly to dimensions of the light pickup. Accordingly, thediameter of the light bundle is a factor which specifies sizes ofoptical parts and determines dimensions of the light pickup. Speakingconcretely, a shorter focal length of the objective lens is morepreferable for configuring the light pickup more small-sized andthinner. Further, the working distance is a factor which restricts aheight of the light pickup and a shorter working distance is morepreferable for configuring the light pickup thinner. Furthermore, it ispreferable in terms of a humidity resistance characteristics to selectglass as a material for the objective lens though it may be made of aglass or plastic material.

Description will be made of an arrangement of the optical systems in thefirst embodiment of the present invention. The optical unit 4 and theoptical unit 8 are arranged so that their optical axes intersect at anangle of nearly 90° at the beam splitter 10. The optical unit 8 isdisposed nearly in parallel with an optical axis of rays which travelfrom the beam splitter 10 to the objective lens 16, whereas the opticalunit 4 is disposed nearly perpendicularly to an optical axis for therays which travel from the beam splitter 10 to the objective lens 16.When a length of an optical path from the light source of the opticalunit 4 to the objective lens 16 is represented by L1 and a length of anoptical path from the optical unit 8 to the objective lens 16 isdesignated by L2, the optical units 4 and 8 are arranged so as tosatisfy the relationship of L1>L2.

FIG. 2 is a sectional view taken along a II--II line in FIG. 1 which isdescriptive of thicknesses of the components described above. In FIG. 2,the reference numeral 13 represents a light beam, or a locus of rayswhich are emitted from the optical unit 4 or the optical unit 8. Thereference numeral 19 designates an erecting mirror which reflects thelight beam 13 so as to align the same with the optical axis of theobjective lens 16.

The objective lens 16 is set to have the focal length of 2.5 mm or lessand the working distance of 1.2 mm or less for the high density opticaldisk, and the optical units 4 and 8 are arranged so as to satisfy therelationship of L1>L2 as described above. Since rays emitted from thelight sources 2 and 6 are divergent, rays are most divergent when theyare incident on the objective lens 16 in the arrangement describedabove. For configuring the light pickup as a whole so as to have adesired size and thickness, an effective diameter of incidence of alight bundle which transmits through the objective lens 16 (a maximumdiameter of a condensing function of the objective lens 16, i.e., avalue a little larger than 2×NA×f (focal length): this applies also tothe following description) is set at 3.0 mm or less in the embodiment (afinite optical system). As a result, it is possible to specify thethickness of an entire light pickup system including periphericalmechanical components for supporting the optical elements and constitutethe entire system so as to have a thickness of 7.5 mm or less asmeasured from a bottom surface of the high density optical disk 17.

Now, shapes of the diffraction grating 5a and the diffraction grating 9awhich are disposed on the optical unit 4 and the optical unit 8, andconfigurations of the light receiving elements will be described withreference to FIGS. 3A, 3B and FIGS. 4A, 4B. FIGS. 3A and 3B show theshapes of the diffraction gratings used in the first embodiment of thepresent invention, whereas FIGS. 4A and 4B show the configurations ofthe light receiving elements used in the first embodiment of the presentinvention.

The diffraction grating 5a shown in FIG. 3A corresponds to the opticalunit 4, and has three divided sections 5b, 5c and 5d as shown in thedrawing. Further, the diffraction grating 9a shown in FIG. 3Bcorresponds to the optical unit 8, and has two divided sections 9b and9c as shown in the drawing.

In FIGS. 4A and 4B, a light receiving element is disposed on a baseplate of the optical unit 4 so as to form four divided light receivingsections at a center and two light receiving sections on each sides ofthe four divided light receiving sections, whereas a light receivingelement is disposed on a base plate of the optical unit 8 so as to formfive divided light receiving sections.

It is preferable to orient the optical unit 4 such that a division lineA which completely divides a semicircular section of the diffractiongrating 5a into two halves is nearly perpendicular to a radial directionof the high density optical disk and mount the light source 2 in such adirection that a longitudinal direction of a far field pattern ofoutgoing rays is in parallel with the radial direction of the highdensity optical disk.

Now, description will be made of reproducing operations of the lightpickup which has the configuration explained above. In this embodiment,a compact disk (hereinafter referred to as a CD) is used as the lowdensity optical disk and a digital video disk (hereinafter referred toas a DVD) is used as the high density optical disk.

First, operations for reproducing information from a DVD will bedescribed. Rays outgoing from the light source 2 with an oscillatingwavelength of 635 to 650 nm transmit through the diffraction grating 5ato be incident onto the beam splitter 10. At least 90% of the incidentrays is reflected by the beam splitter 10 to outgo from the beamsplitter 10 with an optical axis thereof bent approximately 90 degrees.The outgoing rays are incident on the objective lens 16 to form an imageon a DVD 17 owing to a condensing function of the objective lens 16.Rays reflected by the DVD 17 are reflected by the beam splitter 10 aftertransmitting again through the objective lens 16 to be incident on thediffraction grating with an optical axis thereof bent toward the opticalunit 4. The rays incident on the diffraction grating are diffracted bythe three divided regions 5b, 5c and 5d to be incident on the lightreceiving sections formed on the light receiving element 3.

Now, description will be made of a relationship between the raysdiffracted by the diffraction grating 5a and the rays incident on thelight receiving element 3. The rays diffracted by the diffractiongrating 5b (primary diffracted rays) are incident on a light receivingsection formed on the light receiving element 3. This light receivingsection is divided into four sections so as to form regions A, B, C andD. Rays incident on the regions 5c and 5d are incident on the lightreceiving sections 3b and 3c of the light receiving element 3.Description will be made of methods for generating various kinds ofsignals from the rays incident as described above. First, an RF signalis generated by photoelectrically converting the rays incident on thelight receiving sections 3a, 3b and 3c formed on the light receivingelement, converting current signals obtained by the photoelectricconversion into voltage signals, and totalizing the voltage signals.

Then, the so-called hologram Foucault method is used to form a focuserror signal from a differential output between a sum signal in theregions A and C formed on the light receiving element and a sum signalin the regions B and D. On the basis of this focus error signal, anactuator which holds the objective lens 16 is operated in a focusingdirection. Finally, a tracking error signal is generated by convertingvoltage signals from the light receiving sections 3b and 3c into digitalwaveforms by a comparator, and converting a pulse corresponding to aphase difference between the waveforms into an analog waveform throughan integration circuit. On the basis of this tracking error signal, theactuator which holds the objective lens 16 is operated in a trackingdirection.

Now, description will be made of operations for regenerating informationfrom a CD. Rays emitted from the light source 6 with an oscillatingwavelength of 770 to 790 nm are formed into three beams by the beamforming section 9b of the optical unit 8 to pass through the diffractiongrating 9b to be incident on the beam splitter 10. At least 90% of therays incident on the beam splitter 10 are allowed to transmit throughthe beam splitter 10 to outgo from the beam splitter 10 as it is, to beincident on the objective lens 16 to form an image on the CD 18 owing tothe condensing function of the objective lens 16.

Subsequently, rays reflected by the CD 18 transmit again through theobjective lens 16 and the beam splitter 10 to be incident on thediffraction grating 9. The rays incident on the diffraction grating 9are diffracted by the two divided regions 9b and 9c to be incident onfive divided regions E, F, G, H and I formed on the light receivingelement.

Description will be made of methods for generating various kinds ofsignals from the rays incident as described above. An RF signal isgenerated by photoelectrically converting the rays incident on the lightreceiving sections E, F and G formed on the light receiving element 7,converting current signals obtained by the photoelectric conversion intovoltage signals, and totalizing the voltage signals. Then, the so-calledhologram Foucault method is used to form a focus error signal from adifferential output between signals from the regions E and F formed onthe light receiving element 7. On the basis of this focus error signal,the actuator which holds the objective lens 16 is operated in the focusdirection. Finally, a tracking error signal is generated by theso-called three beam method. On the basis of this tracking error signal,the actuator which holds the objective lens 16 is operated in thetracking direction.

The configuration described as the first embodiment is not limiting, andthe locations of the optical unit 4 and the optical unit 8, for example,may be exchanged with each other. Though the light source which emitsthe rays having the wavelengths in the vicinities of 650 nm is used forreproducing information from the high density optical disk and the lightsource which emits the rays having the wavelengths in the vicinities of780 nm is used for reproducing the information from the low densityoptical disk in the first embodiment, the present invention is notlimited to these light sources, and a light source of 650 nm and a lightsource of 400 nm, for example, may be used for the low density opticaldisk and the high density optical disk, respectively. Erecting mirrorswhich reflect rays having wavelengths of 635 to 650 nm and rays havingwavelength of 780 nm may be disposed in an optical path between awavelength filter 15 (see FIG. 5) and the objective lens 16. Further, adeflecting beam splitter which reflects an S deflected component of alaser beam having wavelengths of 635 to 650 nm and transmitsthere-through a P deflected component of a laser beam having awavelength of 780 nm may be used in place of the beam splitter 10.Furthermore, the laser beam of the optical unit may be changed into alaser beam having a short wavelength corresponding to recording orreproduction of information from the high density optical disk.

As understood from the foregoing detailed description, the presentinvention specifies a working distance of 1.2 mm or less. As a result,an effective diameter of incidence of the objective lens 16 which isdetermined in conjunction with a numerical aperture can be set at 3.0 mmor less. This effective diameter makes it possible to specify a size anda diameter of the objective lens, thereby remarkably contributing tosmall-sizing and weight reduction of a light pickup.

When the effective diameter of incidence of the objective lens 16 isspecified, it is additionally possible to specify a maximum diameter ofa finite optical system (since a maximum diameter of divergent rays isthe effective diameter of incidence of the objective lens 16).Accordingly, it is possible to reserve a maximum space for a lightbundle which passes through the objective lens while reserving spacesfor parts of a driving system. Moreover, the light pickup as a whole canbe made 7.5 mm or less thick (a thickness of the light pickup as a wholeas measured from a bottom surface of the optical disk and including theworking distance described above). When the light pickup is configuredto have a thickness of 7.5 mm or less as described above, a disk systemas a whole including a tray section for accommodation/discharge of thelight pickup can be made 12.7 mm (1/2 inch) or less thick, therebyproviding a light pickup which is small-sized, lightweight and usable invarious apparatus.

Then, a second embodiment of the present invention will be describedwith reference to FIG. 5. FIG. 5 shows a configuration of a light pickupaccording to the second embodiment of the present invention. In FIG. 5,the reference numerals 4 and 8 represent optical units, the referencenumeral 10 designates a beam splitter and the reference numeral 16denotes an objective lens. Since the components mentioned above areconfigured and arranged similarly to those used in the first embodiment,they will be designated by the same reference numerals and a detaileddescription thereof is omitted.

Differently from the first embodiment, the second embodiment comprises awavelength filter 15. The wavelength filter 15 transmits therethroughrays emitted from a light source 2 and reflects or absorbs rays emittedfrom a light source 6. Accordingly, the wavelength filter 15 serves as astop which restricts diameters of both light bundles emitted from thelight source 2 and the light source 6.

From viewpoints of enhancement of productivity and cost reduction, it isremarkably preferable that the wavelength filter 15 is disposed betweenthe beam splitter 10 and the objective lens 16 or between the opticalunit 8 and the beam splitter 10, so that it is unnecessary to provide aplurality of filters in correspondence to the individual light sources,and a size of the wavelength filter 15 can be restricted to a minimumsince it is disposed at a location where the light beam has not beenfurther diverged. Such arrangement is most preferable in that thewavelength filter 15 is bonded and provided in a state of having beenpreliminarily positioned relative to the beam splitter 10, therebymaking it possible to reduce a number of members being subjected topositioning at the time of assembly of the light pickup, enhanceproductivity of the light pickup, and minimize deviation between theoptical axis of the rays and a center axis of the wavelength filter.

FIG. 6 is a sectional view taken along a VI--VI line in FIG. 5. In FIG.6, the light beam 13 is narrowed in diameter by the wavelength filter 15(indicated by two-dot chain lines in the drawing) when the light beamoutgoes from the optical unit 8 toward a CD. When a light beam outgoesfrom the optical unit 4 (not shown) toward a DVD, it transmits throughthe wavelength filter 15 (indicated by dashed lines in the drawing). Thelight beams which are divergent light bundles from the two light sourcesare reflected by an erecting mirror 19 to be incident on the objectivelens 16.

As already described with reference to the first embodiment, theobjective lens 16 is set to have a focal length of 2.5 mm or less and aworking distance of 1.2 mm or less for a high density optical disk 17,and the optical unit 4 and the optical unit 8 are disposed so as tosatisfy the relationship of L1>L2. Since the light beams from the lightsources 2 and 6 are divergent, the objective lens 16 is disposed at alocation where the incident light beams are most divergent in thearrangement described above. For the sake of finishing a light pickupsystem as a whole to a desired size and thickness, an effective diameterof light beams transmitting through the objective lens 16 is set at 3.0mm or less also in the second embodiment (a finite optical system). As aresult, it is possible to reserve a maximum space for the light bundleswhich transmit therethrough while reserving spaces for parts of adriving system. Moreover, it is possible to specify a thickness of thelight pickup system as a whole including peripheral members forsupporting the optical elements described above and constitute theentire light pickup system with a thickness of 7.5 mm or less asmeasured from a bottom surface of the high density optical disk 17.

The configuration of the second embodiment described above is notlimitating. For example, in place of the wavelength filter 15, a stopmember (not shown) may be provided between the optical unit 8 and thebeam splitter 10 to have an aperture having such a diameter as tooperate the objective lens 16 within a range of numerical apertures(NAs) from 0.4 to 0.6 for rays from the objective lens 16.

Further, the locations of the optical unit 4 and the optical unit 8 maybe exchanged with each other, and different wavelengths may be selected.

Then, a third embodiment of the present invention will be described withreference to FIG. 7. FIG. 7 shows a configuration of a light pickupaccording to the third embodiment of the present invention. In FIG. 7,reference numerals 4 and 8 represent optical units, the referencenumeral 10 designates a beam splitter and the reference numeral 16denotes an objective lens. Since the components mentioned above areconfigured and disposed similarly to those used in the first embodiment,they will be designated by the same reference numerals and not describedin detail.

Differently from the first embodiment, the third embodiment comprises acollimator lens 14. The collimator lens 14 has a focal length within arange from 11 to 18 mm and a numerical aperture within a range from 0.1to 0.14 . The collimator lens 14 is disposed between a ray outgoingsurface of the beam splitter 10 and the objective lens 16. Thepositional relationship between the collimator lens 14 and light sourceshown in FIG. 7 is such that a light source 2 is disposed at a distanceof a focal length of the collimator lens 14 whereas a light source 6 isdisposed at a distance shorter than the focal length of the collimatorlens 14. In other words, the optical units 4 and 8 are disposed so as tosatisfy a relationship in optical paths of L1>L2 where a length of anoptical path from the light source of the optical unit 4 to thecollimator lens 14 is represented by L1 and a length of an optical pathfrom the light source of the optical unit 8 to the collimator lens 14 isdesignated by L2. As a result, rays emitted from the light source 2 areparallel with one another after transmitting through the collimator lens14, whereas rays emitted from the light source 6 are reduced indivergent angles after transmitting through the collimator lens 14.

Now, description will be made of information reproducing operations ofthe light pickup according to the third embodiment. Operations forreproducing information from a DVD will be described first. As in thefirst embodiment, the rays emitted from the light source 2 are incidenton the beam splitter 10. At least 90% of the rays incident on the beamsplitter 10 is reflected to outgo from the beam splitter 10 with anoptical axis bent at an angle of approximately 90 degrees. The rays areincident on the collimator lens 14, converted by the collimator lens 14from divergent rays into parallel rays, and are incident on theobjective lens 16 to form an image on a recorded data layer of a DVD 17owing to a condensing function of the objective lens 16.

Rays reflected by the DVD 17 are reflected by the beam splitter 10 aftertransmitting again through the objective lens 16 and the collimator lens14, to be incident on the optical unit 4 with an optical axis benttoward the optical unit 4. After the rays are incident on the opticalunit 4, light receiving sections operate to detect and reproduce signalsas in the first embodiment.

Now, operations for reproducing information from a CD will be described.Rays emitted from the light source 6 are formed into three beams to beincident on the beam splitter 10. At least 90% of the rays incident onthe beam splitter 10 is reflected by the beam splitter 10 to be incidenton the collimator lens 14. The rays are converted by the collimator lens14 into rays of reduced divergent angles to be incident on the objectivelens 16 and form an image on the CD 18 owing to a condensing function ofthe objective lens 16.

Rays reflected by the CD 18 are incident on the optical unit 8 aftertransmitting again through the objective lens 16, the collimator lens 14and the beam splitter 10. After the rays are incident on the opticalunit 8, light receiving sections operate to detect and reproduce signalsas in the first embodiment.

FIG. 8 is a sectional view taken along a VIII--VIII line in FIG. 7. InFIG. 8, a light beam 13 outgoing from the optical unit 8 toward the CDis reduced in a divergent angle by the collimator lens 14 (indicated bytwo-dot chain lines in the drawing). A light beam which emerges from theoptical unit 4 (not shown) toward the DVD is converted by the collimatorlens 14 from a divergent light beam into a parallel light beam(indicated by dashed lines in the drawing). Each of the light beams isreflected by an erecting mirror 19 to be incident on the objective lens16.

As described with reference to the first embodiment, the objective lensis set to have a focal length of 2.5 mm or less and a working distanceof 1.2 mm or less for the high density optical disk, and the opticalunits 4 and 8 are disposed so as to satisfy the relationship of L1>L2.Since the light beam emitted from the light source 2 or the light source6 is converted by the collimator lens 14 into the parallel beam orreduced in divergent angle, the light beam 13 from the DVD which hasjust transmitted through the collimator lens 14 is made parallel to havea maximum diameter in the arrangement described above.

For the sake of finishing an entire light pickup system to a desiredsize and thickness, an effective diameter of incidence of a light bundlepassing through the objective lens 16 is set at 3.0 mm or less and adiameter of a parallel light bundle outgoing from the collimator lens 14is set at 3.8 mm or less also in the third embodiment (a finite opticalsystem). As a result, the third embodiment permits specifying athickness of the entire light pickup system including periphalmechanical members for supporting the optical elements described above,thereby making it possible to constitute the entire system with athickness of 7.5 mm or less as measured from a bottom surface of thehigh density optical disk 17.

The configuration described as the third embodiment is not limiting, andfor example, a light source of 650 nm may be used for low densityoptical disks and a light source of 400 nm may be used for high densityoptical disks.

Now, a fourth embodiment of the present invention will be described withreference to FIG. 9. FIG. 9 shows a configuration of a light pickupaccording to the fourth embodiment of the present invention. In FIG. 9,the reference numerals 4 and 8 represent optical units, the referencenumeral 10 designates a beam splitter and the reference numeral 16denotes an objective lens. Since these components are configured andarranged similarly to those used in the first embodiment, they will bedesignated by the same reference numerals and not be described indetail.

Differently from the first embodiment, the fourth embodiment comprises acollimator lens 14 and a wavelength filter 15. Since the collimator lens14 has been described with reference to the third embodiment and thewavelength filter 15 has been described with reference to the secondembodiment, these members are designated by the same reference numeralsand not described in detail.

As described with reference to the second embodiment, the wavelengthfilter 15 is disposed on a ray outgoing surface of the beam splitter 10.Further, the collimator lens 14 has a focal length in the range of 11 to18 mm and a numerical aperture in the range of 0.1 to 0.14 for highdensity optical disks, and is disposed between the wavelength filter 15and the objective lens 16. The collimator lens 14 and light sources aredisposed in such a positional relationship that a light source 2 isdisposed at a distance of a focal length of the collimator lens 14 and alight source 6 is disposed at a distance shorter than the focal lengthof the collimator lens 14. In other words, the optical units 4 and 8 aredisposed so as to satisfy the relationship of L1>L2 where a distance asmeasured from the light source of the optical unit 4 to the collimatorlens 14 is designated by L1 and a distance as measured from the lightsource of the optical unit 8 to the collimator lens 14 is designated byL2.

As a result, the wavelength filter 15 functions as a stop which controlsa diameter of a light bundle, rays emitted from the light source 2 aremade parallel after transmitting through the collimator lens 14 and raysemitted from the light source 6 are reduced in divergent angles aftertransmitting through the collimator lens 14 as described above.

FIG. 10 is a sectional view taken along a X--X line in FIG. 9. In FIG.10, a light beam 13 outgoing from the optical unit 8 toward a CD isrestricted in diameter by the wavelength filter 15 and reduced indivergent angle by the collimator lens 14 (indicated by two-dot chainlines in the drawing). A light beam outgoing from the optical unit 4(not shown) toward a DVD, on the other hand, is converted by thecollimator lens 14 from a divergent light beam into a parallel lightbeam (indicated by dashed lines in the drawing). Each of the light beamsis reflected by an erecting mirror 19 to be incident on the objectivelens 16.

As described with reference to the first embodiment, the objective lens16 is set to have a focal length of 2.5 mm or less and a workingdistance of 1.2 mm or less for a high density optical disk. Further, theoptical units 4 and 8 are disposed so as to satisfy the relationship ofL1>L2 as described with reference to the third embodiment. Since raysemitted from the light source 2 or the light source 6 are converted bythe collimator lens 14 into rays parallel with one another or rays ofreduced divergent angles, a parallel light beam 13 having transmittedthrough the collimator lens 14 toward the DVD has a maximum diameter inthe arrangement described above. For the sake of finishing an entirelight pickup system to a desired size and thickness, an effectivediameter of incidence of a light bundle which transmits through theobjective lens 16 is set at 3.0 mm or less and a diameter of a parallellight bundle which has just transmitted through the collimator lens 14is set at 3.8 mm or less also in the fourth embodiment (a finite opticalsystem). As a result, the fourth embodiment permits specifying athickness of the entire system including peripheral mechanical membersfor supporting the optical elements described above, thereby making itpossible to constitute the entire system with a thickness of 7.5 mm orless as measured from a bottom surface of a high density optical disk17.

The configuration described as the fourth embodiment is not limiting.For example, the locations of the collimator lens 14 and the wavelengthfilter 15 may be exchanged with each other, and the wavelength filter 15in an optical path may be disposed between the collimator lens 14 andthe objective lens. Further, a stop member (not shown) may be disposedbetween the optical unit 8 and the beam splitter 10 in place of thewavelength filter 15, and an aperture of a stop member may be formed tooperate the objective lens 16 within a range of numerical apertures(NAs) from 0.4 to 0.6 for rays from the light source 6. Furthermore, thelocations of the optical unit 4 and the optical unit 8 may be exchangedwith each other.

While the fourth embodiment uses the light source which emits rayshaving a wavelength in the vicinities of 650 nm for reproducinginformation from the high density optical disk and the light sourcewhich emits rays having a wavelength in the vicinities of 780 nm forreproducing information from the low density optical disk, the presentinvention is not limited to this embodiment. For example, a light sourceof 650 nm may be used for the low density optical disk and a lightsource of 400 nm may be used for the high density optical disk.

Now, description will be made of another example of the objective lens16 which has different characteristics. In each of the first throughfourth embodiments described above, the objective lens is usablecommonly for recording and reproducing information from the high densityoptical disk and the low density optical disk, and has a numericalaperture of 0.6 , a focal length of 2.5 mm or less and a workingdistance of 1.2 mm or less for the rays having the short wavelengths forthe high density optical disk.

When an objective lens is made thin with geometric and opticalconditions unchanged, however, a working distance is increased adistance corresponding to the reduced thickness. It is thereforepossible to make the objective lens thin with geometric and opticalconditions unchanged by appropriately selecting a material and athickness of the objective lens on the basis of the facts that the rayshaving the short wavelengths for the high density optical disk use theentire circumference of the objective lens to be focused on therecording layer at the depth of 0.6 mm as measured from the bottomsurface of the optical disk in the condition of NA of 0.6 and that therays having long wavelengths for the low density optical disk use thecentral portion of the objective lens to be focused on a recording layerat the depth of 1.2 mm as measured from the bottom surface of theoptical disk in the condition of NA of 0.45.

As a result, it is possible to set a working distance of 1.4 mm or lessfor the rays having the short wavelengths and a working distance of 1.2mm or less for the rays having the long wavelengths. When the workingdistances are increased, the objective lens is prevented from beingbrought into contact with the bottom surface of the optical disk inabnormal conditions such as a variation of a disk surface due to warp ortwist and a focus control error, whereby it is possible to provide alight pickup which is small-sized, thin and easily controllable.

What is claimed is:
 1. A light pickup for reproducing information froman optical disk recording medium, said light pickup comprising:a firstlight source for emitting light of a first wavelength; a firstphotodetector for detecting light reflected from the optical disk; asecond light source for emitting light having a second wavelength, saidfirst wavelength being shorter than said second wavelength; a secondphotodetector for detecting light reflected from the optical disk; andan objective lens, wherein:said objective lens has an effective diameterof incidence of 3.0 mm or less, a numerical aperture of 0.6 or more anda focal length of 2.5 mm or less for the light having the firstwavelength.
 2. A light pickup as set forth in claim 1 and having anoverall thickness of 7.5 mm or less as measured from a bottom surface ofthe optical disk.
 3. A light pickup for reproducing information from anoptical disk recording medium, said light pickup comprising:a firstlight source for emitting light of a first wavelength; a firstphotodetector for detecting light reflected from the optical disk; asecond light source for emitting light having a second wavelength, saidfirst wavelength being shorter than said second wavelength; a secondphotodetector for detecting light reflected from the optical disk; andan objective lens, wherein:said objective lens has a working distance of1.2 mm or less, an effective diameter of incidence of 3.0 mm or less, anumerical aperture of 0.6 or more and a focal length of 2.5 mm or lessfor the light having the first wavelength.
 4. A light pickup as setforth in claim 3 and having an overall thickness of 7.5 mm or less asmeasured from a bottom surface of the optical disk.
 5. A light pickupfor reproducing information from an optical disk recording medium, saidlight pickup comprising:a first light source for emitting light of afirst wavelength; a first photodetector for detecting light reflectedfrom the optical disk; a second light source for emitting light having asecond wavelength, said first wavelength being shorter than said secondwavelength; a second photodetector for detecting light reflected fromthe optical disk; and an objective lens, wherein:said objective lens (i)has a working distance of 1.2 mm or less for the light having the secondwavelength, and (ii) has a working distance of 1.4 mm or less, aneffective diameter of incidence of 3.0 mm or less, a numerical apertureof 0.6 or more and a focal length of 2.5 mm or less for the light havingthe first wavelength.
 6. A light pickup as set forth in claim 5 andhaving an overall thickness of 7.5 mm or less as measured from a bottomsurface of the optical disk.
 7. An optical disk apparatus having a lightpickup as set forth in claim 1 and having an overall thickness of 12.7mm or less.
 8. An optical disk apparatus having a light pickup as setforth in claim 3 and having an overall thickness of 12.7 mm or less. 9.An optical disk apparatus having a light pickup as set forth in claim 5and having an overall thickness of 12.7 mm or less.